This invention generally relates to inspection apparatus and methods and more particularly relates to an apparatus and method for inspecting a tubular member for degradation, which tubular member may be a control rod drive mechanism penetration tube of the kind typically found in nuclear power reactor pressure vessels.
Although devices and methods for inspecting tubular members are known in the prior art, it has been observed that prior art devices and methods have certain operational problems associated with them which make these devices and methods unsuitable for inspecting control rod drive mechanism penetration tubes of the kind typically found in nuclear power reactor pressure vessels. However, before these problems can be appreciated, some background is necessary as to the structure and operation of a typical nuclear power reactor and its associated control rod drive mechanism penetration tubes.
In this regard, a nuclear power reactor is a device for producing heat by controlled fission of fissionable material contained in fuel assemblies. A plurality of the fuel assemblies are grouped in a sealed reactor pressure vessel to define a nuclear reactor core therein. Pressurized liquid moderator coolant (i.e., borated demineralized water) is caused to circulate through the pressure vessel and over the fuel assemblies for assisting in the fission process and for removing the heat produced by fission of the fissionable material contained in the fuel assemblies.
However, the fission process in the reactor core must be suitably controlled for safety reasons. Therefore, a plurality of vertically movable control rods are each connected to respective ones of a plurality of control rod drive mechanisms (CRDMs) which are connected to the top of the reactor pressure vessel by means of a CRDM penetration tube that penetrates the pressure vessel. Each CRDM vertically positions its respective control rods within the fuel assemblies associated therewith to adjust reactor core reactivity so that the fission process is suitably controlled. The moving internal components of each CRDM, which moves its respective control rods connected thereto, extend through a thermal sleeve mounted in and surrounded by the CRDM penetration tube. The sleeve is freely rotatable and axially movable within the penetration tube. The purpose of the thermal sleeve is to provide a measure of thermal insulation for the penetration tube. The penetration tube itself is welded to the pressure vessel.
Applicants have discovered that, due to degradation of the wall of the penetration tube, through-wall cracks may occur in the tube sufficient to cause leaks of the borated coolant from the reactor pressure vessel. Such a tube leak ultimately may lead to undesirable reactivity anomalies in the reactor core because the inventory of the borated moderator coolant used to assist the fission process is momentarily reduced by the leak. In addition, because the tube forms a portion of the reactor pressure boundary, any such leak may affect the pressure and thus the temperature of the moderator coolant. Variation in the coolant temperature also undesirably affects core reactivity. Such reactivity anomalies are undesirable for safety reasons. It is therefore prudent to inspect the CRDM penetration tubes for any degradation or cracks that might lead to leakage of the coolant. Such an inspection may be attempted by means of an eddy current inspection device that emits electromagnetic energy, or by an ultrasonic inspection device that emits sonic energy, propagating into the penetration tube to detect any degradation in the tube. Such an inspection device is preferably capable of radially and axially inspecting the tube by moving both radially and axially to scan the tube for degradation.
In typical prior art methods of tube inspection, an inspection device is centrally disposed in a tube and then merely moved radially and axially to scan the tube for degradation. However, applicants have discovered that in the case of CRDM penetration tubes, the thermal sleeve, which is concentrically mounted in the CRDM penetration tube, presents a barrier to suitable inspection of the penetration tube. This is so because, in the case of CRDM penetration tubes, centrally disposing the inspection device in the penetration tube also necessarily requires that it be centrally disposed within the sleeve, because the sleeve is concentrically mounted within the penetration tube. Therefore, the sleeve is interposed between the inspection device and the penetration tube and thus presents a barrier to suitable inspection of the tube. This barrier undesirably interferes with the electromagnetic or sonic energy radially emitted and received by the inspection device. This interference is caused by the nonuniform material properties of the sleeve. That is, the "as-built" thermal sleeve may posses nonhomogeneous or nonuniform material properties that vary from nominal design values. Such nonuniform material properties is evinced by radially and axially varying electromagnetic and sonic properties caused by radially and axially varying sleeve density even though the sleeve is nominally formed of a single material (e.g., "INCONEL ALLOY 600"). Moreover, such a sleeve may have radially and axially varying dimensions that differ from nominal values due to variations in the manufacturing process used to make the sleeve. For example, axial portions of such a sleeve may have an oval transverse cross section rather than a perfectly round transverse cross section. Such nonuniform material properties and dimensions interfere with the inspection sensor signals received during the inspection process because the presence of the flaw detected by the inspection device will be undesirably masked by the volumetrically-varying (i.e., nonuniform or nonhomogeneous) material properties of the intervening sleeve. Therefore, a problem in the art is to inspect such a penetration tube, even in the presence of an intervening sleeve having nonuniform material properties and dimensions.
Apparatus and methods for inspecting tubular members for degradation are known. One such apparatus is disclosed in commonly owned U.S. Pat. No. 4,856,337 titled "Apparatus and Method For Providing A Combined Ultrasonic And Eddy Current Inspection Of A Tube" issued Aug. 15, 1989 in the name of Michael J. Metala, et al. Although this patent discloses an inspection device for inspection of a tube, this patent does not appear to disclose an apparatus and method for suitably inspecting a control rod drive mechanism penetration tube of the kind typically-found in nuclear power reactor pressure vessels.
Therefore, what is needed is an apparatus and method for inspecting a tubular member for degradation, which tubular member may be a control rod drive mechanism penetration tube of the kind typically found in nuclear power reactor pressure vessels.